1. Field of the Invention
The invention relates to control systems for automatically adjusting light sources within projectors so as to achieve optimum illumination across the surface on which the light is being projected.
2. Background of the Invention
Present day color film print stock is formulated to give best picture quality when the projection and screen system provide a maximum luminance of approximately 16 ft Lamberts. If less than maximum luminance is obtained, the screen will display a picture with color that appears dull and "washed-out". Conversely if excessive or inconsistent luminance is achieved, some viewers may notice a flicker in the brightness level. It is also important that the luminance is consistent across the entire screen area.
For example, it is well known in the motion picture art that a motion picture image on a film is projected onto a screen when a light passes through the film as it is being fed frame by frame through a projection device. Specifically, as a film is fed through a film projector, each film frame is advanced through a gate in the projector within approximately ten milliseconds. The frame is then held stationary in the projector's gate for approximately 30 milliseconds while the shutter opens twice so that the frame is exposed to two ten millisecond pulses of light. Thus, light pulses at a rate of 48 Hertz are projected through the film through a projection lens and then onto a movie screen to create a visible image. Each new frame is projected every one-twenty-fourth of a second.
A number of factors affect the consistency of the luminance emitted from the light source. Some of the most important factors are the alignment of the lamp (sometimes referred to as the "lamp house") with the projector optics; the adjustment of the light source optics; the luminous power output of the light source; and the quality of the projection lens.
Therefore, in order to obtain the best luminance available from the light source, adjustments of the position of the lamp within the lamp housing in the X, Y and Z axes must be achieved concurrently in a very short period of time.
Presently, there are several devices and methods that are used to measure and adjust the luminance level of the light source. For example, there is known in the prior art a device known as a "spotmeter," which can be used to manually measure the luminance level by taking a reading of the luminance usually at the center of the screen on which the light is being projected. The variations in the luminance level can be measured with the spotmeter by measuring the luminance levels at various other locations on the screen and comparing the measurements with the luminance level at the center of the screen. Any adjustments in the luminance must be made manually.
The spotmeter is similar to a light meter used in photography except that it measures the reflected fight rather than the incident light. The spotmeter is comprised of a photocell attached to a lens that measures the light in a 1.degree. optical path. Thus, it can only view a small area at a time and must be manually moved in order to take measurements over a large area.
Another method used to measure the luminance of the projection system is where a technician uses his naked eye to review the luminance of a screen. However, the human eye is very poor at recognizing or measuring differences in luminance levels.
Another prior art device, the imaging spectroradiometer, described and disclosed in U.S. Pat. No. 5,424,543 issued on Jun. 13, 1995 to Mark Dombrowski, is an instrument that uses a CCD video camera for the purpose of creating spectral images of a scene. Displaying both spatial form and spectral radiance, the spectroradiometer allows acute discrimination of objects within the image. This instrument is designed to provide highly accurate images of a scene for analysis of objects within the scene, but has no means to measure the variation in luminance of motion picture images.
A further prior art device is described and disclosed in U.S. Pat. No. 5,239,174 issued on Aug. 24, 1993 to Raymond Klein entitled "Spectral intensity measuring system for measuring repetitively pulsed light having a multichannel detector array and adjustment system including a digital phase locked loop." In this patent, a measuring system is disclosed that measures the incident light emitted from a light source such as a xenon light, a flourescent lamp or a CRT. It performs this measurement by synchronizing the peak amplitude of a pulsed light source with the amplitude of a multichannel array spectroradiometer before measuring the incident light. The accuracy of the measurement is wholly dependent on the synchronization, such that a tight synchronization must be maintained at all times. While with the proper optics, this prior art system could possibly measure the average reflected light from a movie screen, it would not be able to differentiate between luminance measurements at various locations on the screen.
Since adjustments often need to be made to the lamp-house optics in movie projectors, for example, after replacement of a bulb, recalibration of the luminance pattern using the devices and methods known in the prior art can be a tedious process and often results in a less than optimum light pattern on the screen.
Specifically, in the prior art, the film projector is mounted on a console and carries a lens turret and one or more motors for changing lenses and film aperture. The console which carries the projector includes the projection lamp, reflector, heat filter, lamp power supply, optical system and control panel. The projector has various aperture shapes that are used in connection with present day film. In one arrangement, two apertures are used; one aperture is long horizontally and narrow vertically (sometimes referred to as the Cinemascope or "scope" aperture) and the other aperture is rectangular with a width to height ratio of 1.85 to 1 (sometimes referred to as the "flat" aperture).
The projector usually contains a light source such as a high intensity lamp or the like. Usually, the beam of light from the light source is circular causing some of the lamp's energy to be blocked by each of the apertures. Further, since the projector apertures have different length to width ratios, it is the practice in the prior art to focus the beam to a size which is a compromise between the preferred sizes of each of the two apertures. Alternatively in order to improve the luminance of the light source, the projector operator can manually change the focal point of the lamp. However, changing the focus is not a practical solution to the problem because an operator usually is not present in the projection booth at all times.
Therefore, there has been a long felt need to provide a means to easily, quickly and accurately measure the luminance of the projected light and thereby adjust the light source, so that optimal viewing of the surface onto which the light is being projected may be achieved.
A lamp mounting arrangement that is currently available which can semi-automatically or manually adjust the lamp position thereby causing the lamp focus and beam shape to achieve maximum efficiency for each of the aperture shapes is set forth in U.S. Pat. No. 5,054,909 issued to CHRISTIE INCORPORATED entitled LAMP HOUSING FOR MOTION PICTURE FILM PROJECTOR.
Likewise, the only instrument that is currently available that measures and controls several key performance parameters of motion picture presentations such as overall luminance (brightness of the screen image), low or imperceptible variations in the intensity of the luminance of the screen (otherwise known as "flicker"), and the amount of horizontal and vertical motion of an image that is being displayed on the screen (known respectively as "weave" and "jump" of the image) is set forth in U.S. Pat. No. 5,818,597 issued to Ultra Stereo Labs, Inc. entitled PROJECTION SYSTEM ANALYZER.
Therefore, in order to optimize the luminance of the light source, it would be advantageous to combine some of the features of a PROJECTION SYSTEM ANALYZER such as the one disclosed in the Ultra Stereo Labs, Inc. patent with a MOTORIZED LAMP HOUSING FOR MOTION PICTURE FILM PROJECTOR such as the one disclosed in the CHRISTIE INCORPORATED patent, each modified so that control signals from the PROJECTION SYSTEM ANALYZER automatically control and adjust the position of the lamp, so that optimal luminance and luminance distribution are achieved across the viewing surface.
One of the constraints that applies to automatic adjustment of the lamp is that the screen can only be illuminated for a very short period of time without the film running through the projector gate, The extremely high amount of light energy emitted by the lamp will heat the projector lens. Too much heat can fracture the lens. Thus, any adjustments to the lamp must be accomplished in a very small period of time.